An Etymological Dictionary of the German Language/billig
billig, adj., adv., ‘reasonable (-ably), cheap (-ly),’ for an earlier billich, used even in the last century, from MidHG. billîch, OHG. (recorded since Williram) billîch (adv. MidHG. billîche, OHG. billîhho). ‘conformable, becoming’; cognate with AS. bilewit, MidE. bilewit, ‘simple, innocent.’ It has been said, without sufficient reason, that this class was borrowed from Kelt. Comp. other cognates under Weichbild, Unbill.